1. Field of the Invention
The present invention relates to a multilayer sheet, a pressure-sensitive adhesive sheet, a method for producing a multilayer sheet, and a method for processing articles using such pressure-sensitive adhesive sheet. In particular, the present invention relates to a pressure-sensitive adhesive sheet used in a process of high-precision processing of semiconductor products such as semiconductor wafers and optical products in order to hold or protect such products, to a multilayer sheet used as a base material thereof and a method for producing the same as well as to a method for producing an article using such a pressure-sensitive adhesive sheet.
2. Description of Related Art
In some industrial fields such as optical industries and semiconductor industries, pressure-sensitive adhesive sheets are used in high-precision processing optical parts such as lenses or semiconductor articles such as semiconductor wafers in order to protect the surface of wafers and the like or prevent breakage thereof.
For example, in the process of producing semiconductor chips, the semiconductor chips are produced as follows. First a high-purity silicon single crystal or the like is sliced to form wafers. A predetermined circuit pattern such as IC and the like is formed on the wafer by etching to incorporate an integrated circuit. Then, the back side of the wafer is ground by a grinding machine to a thickness as thin as about 100 to about 600 μm, in some cases to a thickness of 100 μm or less. Finally, the wafer is diced into chips. Since the semiconductor wafer in itself is thin and brittle and the circuit pattern is uneven, the wafer tends to be broken if external force is applied to the wafer when it is transported to the steps of grinding and dicing. In the step of grinding processing, grinding is performed while washing the backside of the wafer with purified water in order to remove the resultant grinding sludge or remove heat generated during the grinding. The grinding sludge and grinding water and the like could cause environmental pollution and it is necessary to prevent such pollution. Consequently, to protect, for example, the circuit pattern surface and prevent breakage of the semiconductor wafer, it has been widely practiced to apply a pressure-sensitive adhesive sheet on the circuit pattern surface of the wafer before the operation can be performed. Further, when dicing, for example, a pressure-sensitive adhesive sheet is applied to the back side of the wafer and the wafer is diced in a glued and fixed state to form a chip. Then, the chip is raised by picking the base material side with a needle and the raised chip is transferred and fixed onto a die pad.
Examples of known pressure-sensitive adhesive sheet used herein include pressure-sensitive adhesive sheets that include a base material sheet made of, for example, polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), or ethylene/vinyl acetate (EVA), having thereon a pressure-sensitive adhesive layer. Japanese Patent Application Laid-open No. 61-10242 discloses a film for use in processing silicon wafers including a base material sheet with a Shore Hardness D of 40 or less having provided on a surface thereof a pressure-sensitive adhesive layer. Further, Japanese Patent Application Laid-open No. 9-253964 discloses a pressure-sensitive adhesive tape that includes a base material made of a radiation-cured composition containing a urethane acrylate-based oligomer and a reactive dilution monomer having provided thereon a pressure-sensitive adhesive layer. Japanese Patent Application Laid-open No. 61-260629 discloses a film for use in processing silicon wafers that includes a base material film with a Shore Hardness D of 40 or less having laminated an auxiliary film with a Shore Hardness D of more than 40 on one surface thereof and a pressure-sensitive adhesive layer on another surface thereof. Japanese Patent Application Laid-open No. 2002-69396 discloses a pressure-sensitive adhesive film for protecting semiconductor wafers, which includes at least three base material films: a low elastic modulus film with a storage elastic modulus (E′) of 1×105 to 1×108 Pa and a thickness of 10 to 150 μm as an outermost layer and a high elastic modulus film with a storage elastic modulus (E′) of 2×108 to 1×1010 Pa and a thickness of 10 to 150 μm as an innermost layer. Japanese Patent Application Laid-open No. 2000-38556 discloses a sheet for protecting semiconductor wafers, which has a hot-melt adhesive layer having a melting point of 105° C. or less. Japanese Patent Application Laid-open No. 2000-150432 discloses a pressure-sensitive adhesive sheet having a stress relaxation rate of 40%/minute or more at 10% elongation in a tensile test.
However, in recent years, differences in height of unevenness on the surface of circuit patterns is increasing and along with miniaturization of chips, semiconductor wafers with a reduced thickness as thin as 100 μm or less are being demanded. For example, a pressure-sensitive adhesive sheet using a rigid base material such as one made of PET is prevented from curling of wafers after they are made thin and polished while the pressure-sensitive adhesive sheet can not follow the unevenness on the circuit pattern on the surface of the wafer, so that the adhesion between the pressure-sensitive adhesive layer and the pattern surface becomes insufficient. This results in peeling of the sheet or penetration of polishing water or foreign matter into the pattern surface. Further, pressure-sensitive adhesive sheets made of a flexible base material such as one made of EVA has no problem with respect to the followability to the pattern surface but because of insufficient rigidity of the base material, such pressure-sensitive adhesive sheets undergo curls that occur after the wafers are polished or deflection due to the self-weight of the wafers. From this, one might consider to use a composite base material obtainable by affixing a PET base material having rigidity and an EVA base material being flexible to each other. However, when the two base materials are mechanically affixed to each other with an adhesive, stress applied when the base materials are affixed to each other remains in the resultant film so that the obtained composite base material will curl. On the other hand, when a laminate is formed from such base materials by a T-die method or by a calendering method, it is difficult to provide a thick film. As a result, films, which are thin, tend to undergo heat shrinking during their film forming process so that residual stress will occur in the films. A pressure-sensitive adhesive sheet produced by using such base materials with residual stress has defects that the wafer is damaged while the wafer is being polished or the wafer curls after its polishing. Furthermore, when a laminate is formed by a solution coating method, use of solvents causes environmental problem as well as it is necessary to repeat coating when a thick film is to be prepared.
Japanese Patent Application Laid-open Nos. 2004-107644 and 2004-122758 disclose pressure-sensitive adhesive sheets that include a composite film containing a urethane polymer and a vinyl-based polymer as major components as an intermediate layer and teach that such pressure-sensitive adhesive sheets are effective in coping with the above-mentioned problems.
A wafer is passed through a dicing step where the wafer is cut into individual chips and the chips are subjected to wire bonding, sealing with a resin and so on. A pressure-sensitive adhesive sheet used in these steps must not contaminate an adherend such as a wafer by an organic substance derived from the pressure-sensitive adhesive or particles a and so on that constitute the pressure-sensitive adhesive layer. However, the above-mentioned pressure-sensitive adhesive sheets are not free of the problem of contamination.
It is known that the contamination on the surface of a wafer gives an adverse influence on the shear strength of wire bonding. That is, in the wire bonding carried out when semiconductor chips are produced, the adhesion strength between a ball and a pad is required to be high, generally organic substances or particles attached on the surface of wafers are factors that prevent a gold wire from bonding to the surface of aluminum. When organic substance or particles that are attached in large amounts on the surface of aluminum, such contamination causes various problems. For example, it serves as a starting point for the formation of voids, with the result that the sealant resin is peeled off from the wafer, cracks occur in the sealant resin, or the shear strength of wire bonding is decreased.
Conventionally, solvent-type acrylic pressure-sensitive adhesives have been used as pressure-sensitive adhesives. Since the solvent-type acrylic pressure-sensitive adhesives are synthesized in organic solvents, use of the pressure-sensitive adhesives causes environmental problem due to the volatilization of the solvent upon coating. For this reason, aqueous dispersion-type acrylic pressure-sensitive adhesives are being increasingly used in place of the solvent-type pressure-sensitive adhesives. However, the aqueous dispersion-type pressure-sensitive adhesives use emulsifiers in contrast to the solvent-type acrylic pressure-sensitive adhesives so that it has been difficult to achieve a low degree of pollution by the pressure-sensitive adhesives.
In particular, along with recent trends of higher density of semiconductor integrated circuits and higher performance thereof, management of contamination onto semiconductor wafers and circuit surfaces of semiconductor chips produced therefrom is becoming severer so that it is required that pressure-sensitive adhesive sheets for use in processing wafers must have much lower contamination than ever.
The present invention has been made with a view to solving the above-mentioned problems and it is an object of the present invention to provide a pressure-sensitive adhesive sheet that is used in processing articles such as semiconductor wafers and gives no adverse effects on semiconductor wafers, for example, breakage of the wafers during polishing even when the wafers to be obtained after the polishing are thin, causes less bending of semiconductor wafers, allows the wafer to undergo less curling due to residual stress of the pressure-sensitive adhesive sheet, and assures low environmental pollution. It is another object of the present invention to provide a multilayer sheet for use as a base material for the pressure-sensitive adhesive sheet. It is still another object of the present invention to provide a method for producing such a multilayer sheet.